Generally pressure vessels and tanks with substantial internal pressure are designed with shape of a complete sphere or a cylinder with doubly curved end enclosures. The main way of carrying the internal pressure in such tanks is by way of membrane stresses in the curved tank walls. Bending stresses in the tank walls are preferably avoided since that reduces the load carrying efficiency for a given wall thickness. A typical trait by membrane type tanks is that the wall stress, and thereby also the wall thickness increases proportionally with the radius of curvature as well as the internal pressure itself whereas the membrane stress is inversely proportional to the wall thickness. For practical reasons, such as practically of welding, the wall thickness has to be limited to a few centimeters for steel tanks. This implies that membrane type shells cannot be made very big when the internal design pressure is large. Another aspect with such pressure vessels is that such tanks cannot be made as a complete double barrier tank without having one complete tank within another complete tank, thereby more than doubling the amount of material required.
The current invention targets tanks that can sustain significant pressures as well as sustaining temperature well below ambient temperature. Low temperature tanks are used for instance for storing Liquid Natural Gas (LNG) both on land as well as onboard ships and offshore installations. Examples of such LNG tanks are cylindrical concrete-steel double barrier tanks for land storage and double barrier membrane and partial double barrier spherical tanks for transportation of LNG onboard ships. Such tanks are not suited significant internal pressure and normally operate at atmospheric pressure. With current attention to the potential environmental advantages by using natural gas for fuel onboard oceangoing vessels there is clearly a need for large fuel tanks of order 1000 to 8000 m3 that can operate with temperatures down to −163 degrees C. and internal pressures of up to 15 bar. These objectives cannot be met with the types of tanks mentioned in the preceding whereas the current invention can meet these requirements as well as even more severe challenges in terms of size, pressure and thermal versatility. Moreover, the current pressure vessel concept can be made double barrier in terms of leak containment as well as double full pressure barrier. It is also easy to insulate the tank on the outside. FIG. 1 is a diagram showing a pressure tank according to the related art, (a) of FIG. 1 is a spherical pressure tank, (b) of FIG. 1 is a cylindrical pressure tank, (c) of FIG. 1 is a lobe-type pressure tank, and (d) of FIG. 1 is a cellular type pressure tank.
The overall efficiency of a tank may be characterized by the volume efficiency and the material ratio.
                    ξ        =                              V            tank                                V            prism                                              [                  Equation          ⁢                                          ⁢          1                ]            
Equation 1 expresses the volume efficiency. Here, ξ represents volume efficiency, Vtank represents the actual volume of the tank, and Vprism represent the volume of an ideal rectangular parallelepiped or prism (brick shape) volume surrounding the tank.
The higher the value of ξ, the better is the storage efficiency of the tank in relation to utilization of the total, brick shaped outer space occupied by one or several tanks. Note that the volume efficiency of a rectangular, prismatic (brick) shape tank is 1.
                    η        =                              V            material                                V            stored                                              [                  Equation          ⁢                                          ⁢          2                ]            
Equation 2 expresses the material ratio. Here, η represents a material ratio whereas Vmaterial expresses the actual volume of the material used for making the tank, and Vstored represents of the gross volume for storing fluid in the tank. p is the internal pressure and σa is the uniaxial, allowable stress. The lower the value of η, the smaller the amount of material is necessary for building the tank in relation to the volume stored, and thus, the better is the structural efficiency of the tank.
TABLE 1Type of Pressure tank  ξ  =            V      tank              V      prism        η  =            V      material              V      stored      Spherical Type0.521.5Cylinder Type0.781.73-2.0Lobe Type0.851.73-2.0Cellular Type<1.01.73-2.0((d) of FIG. 1)
The Table 1 is a table representing the volume efficiency and the material ratio of the tank according to the related art. Note that the material used for the end capping of the cylindrical, lobe and cell type tanks are not included. Moreover, the best material performance is obtained when assuming that the deviatory stress criterion applies (von Mises stress) in connection with allowable stress; this is due to that the hoop stress in these tanks is exactly twice the longitudinal stress.
As seen from the table the spherical tanks have the best material performance; unfortunately, their volume efficiency is very poor. This means that it is not possible to utilize a high portion of a given outer, surrounding volume for actual storage within a series of spherical tanks.
As can be appreciated from the Table 1, the cellular type tank has the most efficient volume efficiency and the material ratio has a value similar to the cylindrical type tank, the lobe type tank, and the cellular type tank.
However, since the lobe type tank is manufactured by intersecting circular tank with each other as well as with cylindrical and planar tank walls, it is difficult to manufacturing such type of tank. High stresses will typically be concentrated at the intersecting lines between internal bulkhead, cylindrical parts and doubly curved parts, which may greatly reduce the material efficiency of such tanks. In practice it is not possible to make a high pressure lobe tank as a double barrier tank because of geometrical complexity.
The cellular type tank has high volume efficiency because of the repetitive cells in two directions. Its material ratio is also good in that it corresponds to that of cylindrical tanks. A main drawback with cellular tanks is that it is difficult to design good ways of closing the ends of the cells without creating significant local bending deformations and stress concentrations. Further, there is a problem in that it is difficult to form the outer wall of the cellular type tank as a double wall in connection with a design.